There are many processes known in the art for converting hydrocarbon sources such as natural gas, coal, coke, into more valuable hydrocarbon products. A typical conversion process involves first converting the hydrocarbon source into synthesis or syngas gas, which is a mixture of water, carbon dioxide, carbon monoxide and hydrogen. If the hydrocarbon source is natural gas, a catalytic reforming reaction is utilized to make syngas. If the source is residual oil or a solid feed, partial oxidation or gasification is used. The syngas then may be used as a feedstock for producing a wide range of chemicals, including combustible liquid fuels, methanol, ammonia, acetic acid, dimethyl ether, oxo alcohols, and isocyanates.
Remote natural gas assets can be converted into conventional transportation fuels, chemical feedstocks, and lubricants via the initial production of syngas. The Fischer Tropsch process is the conventional route to convert the syngas into transportation fuels and lubricants. Alternatively, natural gas may be converted into syngas followed by methanol synthesis with the methanol utilized to produce a wide variety of chemicals.
In particular, a Fischer Tropsch synthesis reaction may be used to synthesize higher molecular weight hydrocarbon products from synthesis gas. In Fischer Tropsch synthesis reactions, synthesis gas is converted to hydrocarbons by contact with a Fischer Tropsch catalyst under reactive conditions. The products from a Fischer Tropsch process may range from C1 to C200+ with a majority in the C5–C100+ range. The Fischer Tropsch synthesis reaction can be conducted in a variety of reactor types including, for example, fixed bed reactors containing one or more catalyst beds, slurry reactors, fluidized bed reactors, or a combination of different type reactors.
Similarly, methanol can be produced from a wide range of hydrocarbon feedstocks by initially converting the feedstock into synthesis gas by reforming or gasification. Methanol synthesis can then be achieved via a catalytically-enhanced reaction.
Hydrocarbon synthesis processes typically require a source of hydrogen gas for use in the process. By way of example, natural gas feeds to the synthesis gas generator may require hydrotreating prior to introduction into the synthesis gas generator. In addition, Fischer Tropsch products, while they are highly paraffinic, are typically upgraded by one or more hydroconversion processes to provide more valuable products. Examples of hydroconversion processes include hydrotreating, hydrocracking, hydroisomerization, and hydrofinishing. In these hydroconversion/hydrotreating processes, expensive hydrogen gas is consumed.
Conventional sources of hydrogen gas are expensive. The hydrogen gas may be obtained from a conventional steam methane reformer; however, the equipment is expensive and the process requires a natural gas feed, which instead could be used to generate additional syngas and ultimately more valuable higher molecular weight products. Hydrocarbon synthesis processes generate gas streams comprising low concentrations of hydrogen. However, the hydrogen is in such low concentrations that typically, these gas streams are sent to fuel gas systems.
As hydrocarbon synthesis processes require hydrogen gas and typical sources for hydrogen gas are expensive, there have been attempts to develop more economical and efficient sources of hydrogen gas for use in these hydrocarbon synthesis processes. By way of example, U.S. Pat. Nos. 6,043,288 and 6,103,773 and WO 02/051744 describe processes for producing hydrogen from a synthesis gas feed. U.S. Pat. No. 5,082,551 describes a process for the separation of H2-rich gas from the effluent from a hydroconversion zone and U.S. Pat. No. 6,147,126 describes processes for using the H2-rich tail gas from a hydroconversion process for at least one of (i) hydrocarbon synthesis reaction catalyst rejuvenation, (ii) the hydrocarbon synthesis, and (iii) hydrogen production. U.S. Pat. No. 5,844,005 describes using a hydrogen containing tail gas from a hydrocarbon synthesis reactor as a hydrogen containing catalyst rejuvenating gas. If CO is present in the hydrogen containing tail gas, the CO content is less than 10 mole % of the gas and the H2 to CO mole ratio is greater than 3:1.
Accordingly, there is a need in the art for an economical and efficient source of hydrogen gas. As such, there is a need in the art for processes for providing high purity hydrogen gas from gas streams comprising low concentrations of hydrogen. In addition, there is a need in the art for a hydrocarbon synthesis process in which the hydrogen required for hydrotreating the natural gas feed and/or upgrading the products is economically and efficiently provided from the hydrocarbon synthesis process itself such that the use of hydrogen from an outside source is minimized. This invention provides such processes.